Field of the Invention
The invention relates to a matting agent comprising agglomerates of pigment particles, to a method for the preparation of such matting agents, and to coating formulations containing the matting agents disclosed herein. The present invention further relates to a plastic containing such matting agents, to pigmented mat surfaces, and to the use of agglomerates of pigment particles for matting pigmented coatings.
Technological Background of the Invention
Surfaces of plastics, laminates and coatings, such as paints and printing inks, are conventionally matted by selective roughening in the micro range. The light impinging on the roughened surface is consequently scattered diffusely rather than directionally reflected. The height, shape and number of the surface structures determine the degree of gloss of the coating, of the paint film, or of the printing ink.
Specific matting agents are usually added to achieve a mat surface. Such matting agents include, for example, spherical particles with a particle size within a range of from 1 to about 20 μm, as a function of the film layer thickness. Other usual matting agents are based on precipitated or pyrogenic silicas or silica gels, but also matting agents based on duromers, waxes or thermoplasts. Further known is the suitability of silicates, such as diatomaceous earth and calcined china clay, as well as calcium carbonate and different kinds of hollow spheres.
EP 1 398 301 B1 discloses matting agents for paints and lacquers based on precipitated silicas having particle sizes (d50) within a range of from 5 to 15 μm and a specific surface (BET) of from 350 to 550 m2/g. The silicas are optionally coated with wax.
Further, DE 195 16 253 A1 describes matting agents based on spray-dried silica gel. In order to ensure sufficient stability of the aggregates, a binder, such as a layer silicate, pyrogenic silica or organic polymer, is used. The agent has a particle size of from 1 to 20 μm and a specific pore volume of from 0.4 to 2.5 mL/g.
Finally, DE 1 519 232 A1 discloses a mat coating containing pigment agglomerates prepared by the spray-drying and subsequent annealing of commercially available pigments together with sodium silicate as a binder. The agglomerates have diameters of from 2 to 70 μm.
Conventional matting agents are incorporated in coating formulations. The latter are applied to substrate surfaces in order to mat the surface. Due to known matting agents contained, the formulations have significant disadvantages in the preparation and handling thereof. Inter alia, established agents tend to dust. Further, the agents are mechanically comminuted when incorporated in coating formulations. This leads to a significant increase in viscosity of the formulation, which makes the production, handling and processing more difficult. Coatings containing conventional matting agents are chemically unstable, susceptible to soil, and exhibit low abrasion resistance. This results in a quick wear and unaesthetic appearance of the mat surface.
Therefore, there is a need in the art for a matting agent that overcomes the drawbacks of the prior art at least in part.